This invention relates to a method and apparatus for the continuous crystallization of sugar from a sugar bearing solution.
In the production of granular sugar from sugar beets and the like, the sugar must be separated from the plant tissue and then placed in a marketable, granular form. This is commonly accomplished by the steps of slicing the sugar beets into thin sections or "cossettes", allowing the sugar and some impurities to diffuse out of the plant tissue into surrounding diffusion juice, purifying the diffusion juice to form a "thin juice", evaporating the thin juice to form a "thick juice" and then crystallizing sugar from the thick juice.
Prior to crystallization, the thick juice is commonly "enriched" with remelt (redissolved) sugar and wash water to form a standard liquor solution. Since the standard liquor contains both sugar and non-sugar portions, the sugar must be separated from the non-sugars, usually by crystallization of the sugar from a supersaturated standard liquor solution. Supersaturation of the sugar in the solution is typically obtained by boiling the standard liquor under controlled conditions. To avoid burning, carmelization and invert sugar formation, boiling of the standard liquor is commonly carried out at a relatively low temperature and pressure in an apparatus known as a vacuum pan. A charge of standard liquor is taken into the vacuum pan and boiled until the liquor is supersaturated, i.e., until it contains a greater concentration of sugar in solution than the normal solubility of sugar in water at that temperature. At this point, sugar crystal nuclei are either permitted to form spontaneously, or the liquid is "shocked" by the injection of a controlled quantity of sugar seed crystals. By careful control of the condition of supersaturation through control of internal pressure, temperature, and solution density, the sugar crystals are permitted to grow until they reach a desired size. The mixture of standard liquor and crystallized sugar, known as massecuite, is then transferred to a mixer and then to centrifugal separation apparatus which separate the sugar crystals from the surrounding liquor. The sugar crystals are then washed with hot water, dried and processed for packaging.
Conventional vacuum pans for use in conjunction with the aforedescribed process are operated on a batchwise basis and require the use of experienced individuals, known as sugar boilers, who manually control the operating variables. Since the crystallization process is highly dependent upon such factors as temperature, pressure, purity of the standard liquor, degree of supersaturation, etc., manual operation of vacuum pans can result in a significant risk of operator error and wide variation between batches of sugar so processed. To overcome these problems, and to increase the efficiency of the crystallization process, it has previously been proposed to carry out crystallization of sugar on a continuous basis. For example, U.S. Pat. Nos. 2,160,533; 2,346,517; 2,587,293; 2,743,198; 3,424,221; 3,505,111; 3,506,486; 3,554,800; 3,556,845; 3,627,582 and 3,879,215 relate to such continuous processes. Prior art continuous crystallization processes and apparatus have dealt primarily with crystallization in multi-stage crystallization apparatus. Such approaches have met with limited success and have required relatively large capital expenditures in the conversion from single stage batch vacuum pans. Furthermore, due to varying quality of sugar beets and varying purity of diffusion juice in different segments of a sugar beet processing season, attempts to crystallize sugar on a continuous basis have heretofore been impractical.
It has now been discovered that a unique continuous process of sugar crystallization can be carried out, with significant economical savings, by a process and system which, in one embodiment, incorporates a single-stage, continuous crystallization system comprising a vacuum pan for continuously boiling a standard liquor solution at a regulated temperature and pressure, purity control means for regulating the ratio of components in the standard liquor in order to obtain a standard liquor solution having a constant, desired purity, standard liquor solution supply means for supplying the standard liquor solution of desired purity to the vacuum pan at a regulated rate, density control means for regulating the density of the standard liquor in the vacuum pan, seed crystal supply means for continuously supplying seed crystals to the vacuum pan at a constant, predetermined rate, a massecuite mixer for receiving a portion of the massecuite from the vacuum pan and maintaining sugar crystals in suspension in the massecuite, and a continuous centrifugal separation means for receiving massecuite from the massecuite mixer and separating the sugar crystals having at least a predetermined desired size from the surrounding liquor of the massecuite. The seed crystals are preferably introduced into the vacuum pan at a point above the boiling massecuite.